1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine having a power switch for intermittently feeding a primary current to an ignition coil which is integrated with the power switch.
2. Description of the Related Art
FIG. 5 is a side elevational view of a conventional ignition apparatus for an internal combustion engine, FIG. 6 is a cross sectional view taken along line VI--VI of FIG. 5 and FIG. 7 is an electric circuit diagram of the ignition apparatus for the internal combustion engine. In these figures, a hole 1b is defined to the bottom of the accommodating section 1a of a synthetic resin insulating case 1 at the center thereof. The insulation case 1 accomodates an ignition coil 16 comprising a primary coil 4 and a secondary coil 6. The iron core 2 of an ignition apparatus having a gap 2a extends through the hole 1b. The primary coil 4 is disposed to surround a portion of the iron core 2. The primary coil 4 is arranged such that a primary coil winding 4a is wound around a primary bobbin 3. A secondary bobbin 5 is disposed around the outer periphery of the primary coil 4 of the ignition coil and a secondary coil winding 6a of the secondary coil 6 is wound around the secondary bobbin 5.
A power switch 7 is accommodated in an auxiliary accommodating unit 1c projecting from the accommodating unit 1a. The power switch 7 is composed of a power transistor 8 and an integrated circuit (IC) for driving and controlling the power transistor 8 which are sealed by a resin as solid members by insert molding. A heat sink 9 is fixed on the upper surface of the power switch 7 by screws 10.
The insulation case 1 includes holding portions 1d, 1e for a high-tension terminal as the high-tension unit of the ignition coil and a holding portion 1f for a low-tension terminal unit 11. A connector housing 1g is engaged with the holding portion 1f of the low-tension terminal unit 11. Note, the connector housing 1g is omitted in FIG. 5. There is disposed in the holding portion 1f a low-tension terminal assembly 12 which is integrated with the low-tension terminal unit 11 composed of a power supply terminal 11a, a signal terminal 11b and a ground terminal 11c.
The primary coil winding 4a has an end 4a.sub.1 which is wound around the tentative fastening portion 3a of the primary bobbin 3 and then the extreme end of which is wound around the power supply terminal 11a several times and soldered thereto. The other end 4a.sub.2 of the primary coil winding 4a is wound around a tentative fastening portion 3a and then the extreme end of which is wound around the terminal 7a of the power switch 7 and soldered thereto. The signal terminal 11b is electrically connected to the terminal 7b of the power switch 7 through a conductive wire 13a. This electric connection is effected such that both the ends of the conductive wire 13a are wound around the signal terminal 11b and the terminal 7b of the power switch 7 several times, respectively and then soldered thereto. The ground terminal 11c is electrically connected to the terminal 7c of the power switch 7 through a conductive wire 13b. Both the ends of the conductive wire 13b are also wound around the ground terminal 11c and the terminal 7c of the power switch 7 several times and then soldered thereto. The secondary coil winding 6a has an end connected to a high-tension terminal 14 and the other end connected to a high-tension terminal in the holding portion 1d of the secondary winding 6a. Note, the accommodating portion 1a and the auxiliary accommodating portion 1c of the insulation case 1 are filled with an insulating resin material 15 composed of a cured epoxy resin.
In the ignition apparatus for the internal combustion engine arranged as described above, the power switch 7 intermits a primary current flowing from the power supply terminal 11a to the primary coil 4, the power switch 7 and the ground terminal 11c in response to a signal input from an external control unit to the signal terminal 11b. When the primary current is shut off, a high-tension voltage is generated at the secondary coil 6 and output to an external ignition plug through the high-tension terminal 14.
Next, a sequence for assembling the ignition apparatus for the internal combustion engine arranged as described above will be described. First, an end of the primary bobbin 3 around which the primary coil winding 4a is wound is fixed to the surface of the insulation case 1 by ultrasonic fusion and then the secondary coil 6 composed of the secondary coil winding 6a wound around the secondary bobbin 5 is tentatively placed to the insulation case 1. Further, the power switch 7 to which the heat sink 9 is fixed by the screws 10 and the low-tension terminal assembly 12 are disposed in the auxiliary accommodating portion 1c.
Next, after the insulation coating of the both ends 4a.sub.1, 4a.sub.2 of the primary coil winding 4a wound around the tentatively fastening portion 3a are removed at the extreme ends thereof, the primary coil winding 4a is tentatively fastened to the power supply terminal 11a and the terminal 7a of the power switch 7 by hand at the extreme ends thereof, respectively. The conductive wires 13a, 13b are also tentatively fastened to the terminals 7b, 7c of the power switch 7, the signal terminal 11b and the ground terminal 11c by hand at both the ends thereof, respectively. Thereafter, the tentatively fastened portions are soldered, respectively.
Next, the connector housing 1g is mounted to the auxiliary accommodating portion 1c. This mounting job is carried out after the power supply terminal 11a, signal terminal 11b and ground terminal 11c are tentatively fastened and soldered to prevent the mounting job from interrupting the fastening and soldering jobs.
Next, the iron core 2 is mounted to the primary bobbin 3, then the insulation case 1 is filled with the insulating resin material 15 with its opening side facing upward and the insulating resin material 15 is heated and cured in a furnace.
In the conventional ignition apparatus for the internal combustion engine, when a primary current is fed to the power transistor 8 of the power switch 7, a voltage drops between the collector and the emitter of the power transistor 8 and heat is generated. Unless the heat is properly radiated, the power transistor 8 fails. To cope with this problem, the heat sink 9 must be in contact with the radiating portion of the power switch 7 under a sufficient pressure and the screws 10 are used as tightening devices for this purpose. Thus, there is a problem that the tightening devices are required, a manday is increased for a tightening job and a manufacturing cost is increased accordingly.
There is also a problem that a space for accommodating the heads of the screws 10 is necessary, the thickness of the heat sink 9 must be increased to a certain extent to cut screw threads to it and thus the size of the auxiliary accommodating portion 1c of the insulation case 1 is increased.
After the primary coil 4, secondary coil 6 and low-tension terminal assembly 12 are placed in the insulating case 1, respectively, the job for tentatively fastening the one end 4a.sub.1 and the other end 4a.sub.2 of the primary coil winding 4a and the conductive wires 13a, 13b at respective predetermined locations and soldering them must be carried out by hand, thus there is a problem that a working time and a manufacturing cost is increased.
Further, there is also a problem that a working space is required for the tentative fastening job and soldering job, the area of the opening of the insulation case 1 is increased and the size of the apparatus cannot be reduced as a whole.